The technical demands on micromechanical components are growing. Beside highest precision, there is a need to provide highest energy efficiency in the mechanical system, long lifetime and complete abdication of lubricants insofar as possible.
In the last few years, a lot of documents dealing with this subject have been published. The described approaches fulfil some of the tasks without being able to deliver a complete solution mainly due to the restrictions on the materials used.
Micromechanical components produced by mechanical machining (die cutting or shape cutting) exhibit two major disadvantages. First, they are either high priced or economically profitable only in mass production because investments are required for expensive production tools. Second, these processes reach their technical limits with an accuracy level lying by +/−5 micrometers.
Thus several alternative approaches have been discussed in literature. One of the most promising idea relates to the etching of micromechanical parts from silicon wafers achieving highest precision, even exceeding the results of the machining techniques by far. The tolerances can be reduced to the sub-micrometer range, but at the expense of lifetime: in practical results it was shown that the mechanical strength as well as the abrasive wear of these parts could not meet the demands in the absence of lubricant. One solution to the problem has been demonstrated in EP patent No 1 904 901: by treating the surface of the micromechanical parts with oxygen the strength and the lifetime could be extended, but without achieving an ultimate solution however.
The tribological performances could be enhanced by using special oils in mechanical systems, however at the expense of the demand for a dry running system.
The longest lifetime is achieved by classical machined parts made of steel, but these systems have reached their limits in regard to high accuracy and have to be further lubricated.
A further problem of the lubricated systems is the necessity for frequent service intervals, where the movements have to be cleaned and re-lubricated.
Thus the cycles of operation are limited and additional costs arise. These service intervals are necessary due to aging of the used oils which are loosing their properties with time.
Numerous approaches to fulfil all these requirements with one system have been undertaken.
In EP0732635B1 an approach is described where the micromechanical parts are etched from a silicon wafer and then coated with diamond films. The diamond films obtained via this method have a surface roughness higher than 400 nm. These films require therefore a subsequent polishing if the diamond coated parts are used in sliding contact applications.
EP1233314 discloses a mechanical clockwork assembly for watches having a mechanical escapement with an escapement wheel and an anchor wherein the functional elements of the escapement wheel are at least partially coated on their operating surfaces with a DLC (diamond like carbon) coating. DLC has a high sp2 content (ranging from 30-100%) and is amorphous carbon which hardness is not sufficient for effective wear protection applications.
EP1622826 discloses a micromechanical component comprising a first surface and a second surface, which are substantially perpendicular in relation to one another wherein the first and/or the second surface consist at least partially of diamond.
U.S. Pat. No. 5,308,661 discloses a process for the pre-treatment of a carbon-coated substrate to provide a uniform high density of nucleation sites thereon for the subsequent deposition of a continuous diamond film without the application of a bias voltage to the substrate.
EP1182274A1 discloses a method for the post-treatment of diamond coatings where a coarse-grained (micrometer regime) diamond coating is deposited on machining tools and subsequently treated be means of plasma processes. The aim of this post-treatment is the degradation of the top layers of the sp3-hybridised diamond coating into sp2-hybridised carbon species. The expectation is a filling of the “surface valleys” between the coarse grains protruding from the surface to achieve a more flat surface. The result of such method is a film having a coarse grain sp3 diamond on top of which is a top layer of several hundred nanometers of sp2 hybridised amorphous carbon. The top layer is relatively soft and will wear off quickly in applications involving high friction.
All the solutions described above can solve only partly the problem of providing micromechanical components featuring coefficient of friction lower than 0.05, and thus a large-scale production was prevented which is however demanded in the watch industry for instance.
In particular, when using diamond-coated silicon, the solutions described above raised the following problems: the diamond coated micromechanical components often exhibit a high initial coefficient of friction because of the microcrystalline structure of the diamond coatings. This high coefficient of friction severely limits the efficiency of the micromechanical system during the first hours of its life.
It is well known that surfaces with a roughness above several hundred nanometers cannot achieve straightforwardly low coefficients of friction. Moreover, utilising coarse diamond films in mechanical systems require a very smooth counterpart. In such cases the rough diamond film would grind into its corresponding counterpart leading to a very quick abrasive wear of the system and its breakdown.
In theory, a special case is imaginable, where different roughness modules are fitting special conditions and therefore produce a low coefficient of friction. However the pressure on each single grain would be too high leading to a breaking and/or interlocking of the grains. The mechanical system would thus lose its properties quickly ending up in a high coefficient of friction and thus a blocking of the system. After the breakdown of the coating, the whole system would collapse and/or damage the whole watch.
Solutions suggesting a polishing after the diamond-coating and therewith a smoothening of the surface of the micromechanical components failed because of high costs, the low efficiency and an essential technical reason: the most important functional surfaces are the flanks of the micromechanical parts which are not accessible for mechanical polishing when mounted in a wafer. A polishing after removing the parts from the wafers is not easy and moreover uneconomic because of the multitude and the diminutiveness of the micromechanical parts. Solutions with plasma etching of a diamond coated wafer comprising the micromechanical parts have also failed due to non-homogeneities of the plasma polishing especially on the flanks of the parts which are the most important areas (see above).
Approaches using smaller crystal sizes (few hundreds nanometers) suffered similar problems in smaller dimensions. For instance, plasma etching of the flanks is not feasible because this process affects mainly the grain boundaries and etches the surfaces in an anisotropic way.
Additionally, anisotropy of the etching treatment can arise from several parameters. The etching efficiency depends strongly on crystallographic orientations of the diamond crystals. As diamond films grown on substrates other than diamond (silicon in most cases) exhibit a mixture of crystallographic orientations, then etching is non-uniform, which can even increase the surface roughness of diamond instead of decreasing it.